There are more horses’ asses than horses.

The earliest located instance (1931) appears in a Chicago sporting periodical, where the writer refers to it as “Uncle Joe” Cannon’s saying while discussing horse racing officials. Earlier jokes existed with a similar structure (“more asses than horses”), but without the specific “horses’ asses” phrasing.

A wordplay insult: it treats “horse’s ass” as a category of foolish people and jokes that they outnumber actual horses, implying incompetence is widespread (especially in the setting being criticized).

“…the truism of Old ‘Uncle Joe’ Cannon’s saying, that peculiar as it may seem, ‘there are more horses asses in the world than horses.’”

There are more asses than horses.

There are more horses’ asses than horses at a track.

There are more horses’ backsides in the military service of the United States than there are horses.